1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display device, and more particularly, to an OLED display device that displays a white color using complementary first and second colors.
2. Description of the Related Art
A flat panel display device (FPD) has been employed as a display device to replace a cathode-ray tube (CRT) display device because the FPD is lightweight and thin. Typical examples of the FPD are a liquid crystal display (LCD) and an organic light emitting diode (OLED) display device. In comparison with the LCD, the OLED display device has a higher luminance and a wider viewing angle and can be made ultrathin because the OLED display device needs no backlight.
The OLED display device includes an organic layer having at least one organic emission layer (organic EML), which is interposed between an anode and a cathode. Thus, when a voltage is applied between the anode and the cathode, electrons injected through the cathode and holes injected through the anode recombine in the organic EML to produce excitons. As a result, light with a specific wavelength is generated due to the energy of the excitons. The organic layer may further include a hole injection layer (HIL), a hole transport layer (HTL), a hole blocking layer (HBL), an electron transport layer (ETL), and/or an electron injection layer (EIL) to facilitate the injection and/or transport of the electrons and holes.
The OLED display device may be classified into a passive matrix type and an active matrix type depending on how the N×M pixels of the OLED display device that are arranged in a matrix are driven. An active matrix OLED display device includes a circuit using a thin film transistor (TFT). A passive matrix OLED display device can be fabricated using a simple process because anodes and cathodes are arranged in a matrix on a display region. However, the passive matrix OLED display device is applied only to low-resolution small-sized display devices due to its resolution limit, its needs for a high driving voltage, and its short material life span.
By comparison, in the active matrix OLED display device, a TFT is disposed on each pixel of a display region. Thus, a constant amount of current can be supplied to each pixel so that the active matrix OLED display device can emit light with a stable luminance. Also, since the active matrix OLED display device consumes low power, the active matrix OLED display device can be applied to high-resolution large-sized display devices.
In an OLED display device, when light is emitted from an organic layer at or greater than a critical angle, the light emitted from the organic layer is totally reflected at an interface between a layer having a high refractive index and a layer having a low refractive index. Thus, light extraction of the OLED display device is only ¼ light emitted by the organic layer.
In order to solve this problem, the thickness of the organic layer may be varied according to the color of light emitted by the organic layer, or a refractive layer may be formed in a direction in which light emitted by the organic layer is extracted, so that a resonance effect may occur due to an optical micro-cavity to thereby improve the light extraction efficiency of the OLED display device.
However, an organic layer of an OLED display device, for displaying a white color using complementary colors, includes first and second EMLs for displaying first and second colors. Because the first and second EMLs are sequentially stacked, it is difficult to improve the luminous efficiency and luminance of a displayed white color using a resonance effect caused by an optical micro-cavity, and, as such, color coordinates are degraded.